Durable outbox with stable idempotency keys

[longevityboris]

Problem

Sends use a random UUID as idempotency key per call. If the network fails after Resend accepts but before the CLI records the result, a retry generates a new UUID and produces a duplicate send.

Proposal

1. Add outbox table: store send intent with a stable idempotency key before calling API
2. Key derived from content hash (from + to + subject + body hash) so retries reuse the same key
3. Resend honors idempotency keys for 24 hours
4. Add outbox list and outbox retry commands for manual recovery
5. Offline sends write to outbox; outbox flush sends when online

Priority: HIGH — data loss risk on unreliable networks